Studies of transfusion-associated hepatitis at NIH and of community- acquired at CDC indicated that 10%-20% of observed hepatitis cases could not be accounted for by the known hepatitis viruses: HAV, HCV, HDV, and HEV. To investigate the existence of an additional agent or agents of human hepatitis, a CRADA was established between the Department of Transfusion Medicine, NIH, Genelabs Technologies, Inc., Emeryville, CA, and CDC. Suspect clinical materials were sent to Genelabs, and they were amplified following reverse transcription by Sequence Independent Single Primer Amplication and then cloned into E. Coli using a gt1 expression vector. A unique clone was identified that was novel to the gene bank and exogenous by hybridization assays. The agent could be recovered from the cloning source and was shown to be a flavivirus distinct from other members of that family. It had approximately 25% homology with HCV. A PCR assay was developed and clinical materials screened. The agent, tentatively designated HGV, accounts for approximately 20% of transfusion and community-associated hepatitis that is unrelated to previously recognized agents. HGV has been shown to cause acute and chronic hepatitis. It is found in 1-2% of blood donors and has been proven to be transfusion-transmitted by linked donor-recipient samples. It appears also to be the causative agent on some cases of hepatitis-associated aplastic anemia.